One way to permanently and electrically program bi-polar integrated circuits is to use so-called Zener-zap of NPN-transistors. The NPN-transistor can be programmed by forcing such a strong current through the transistor so that aluminium from the interconnect pattern migrates into the silicon and forms a short-circuit between the base and the emitter.
The NPN-transistor can be used as a logical one or a logical zero when it is zapped, depending on the connected logic. Thus, a sequence of initially non-zapped NPN-transistors can be used to represent a sequence of binary numbers, representing a digital number. This sequence may then be altered, by zapping selected binary numbers in the sequence, to change the digital number represented by said sequence of NPN-transistors.
As an example, the operating point for a power transistor may initially be set to a predetermined value in an integrated circuit. After testing and tuning it is possible that the operating point should be changed slightly to achieve better performance. If the predetermined value for the operating point is set in the integrated circuit using NPN-transistors this value may be changed later, after tuning, by zapping carefully selected NPN-transistors.
It is possible to connect the NPN-transistor in two different ways to produce a Zener diode, namely with the collector connected either to the base or to the emitter. The collector-base connection requires less voltage to zap the Zener diode but the consumed current is considerably larger. If used as a memory function the collector-emitter connection is preferable since less current is consumed. If larger memories should be programmed the current requirements could otherwise be excessive. This alternative has, however, the draw-back that a larger voltage is required.
For circuits involving many components small dimensions are required. These components, having small dimensions, are less tolerant to high voltages. That is, they cannot sustain high voltage without being damaged. It would be preferable to be able to program many NPN-transistors simultaneously also in circuits involving many, and thus small, components, which are incapable of sustaining high voltages.